Mendel90 extruder

The Mendel90 parametric design starts from the extruder dimensions and works outwards. I used a Wade's extruder for the Mendel sized version of the machine (I will need to sort out a smaller extruder for the Huxley sized version). My starting point was the Prusa version of Wade's. I tidied it up a bit aesthetically and made a few tweaks to the design and that had the side effect of making it easier for Skeinforge to slice correctly. The old version caused it to think layers were bridges erroneously. It now looks like this: -

The functional things I tweaked were: -

• I added nut traps for captive hex head bolts. That allows me to fasten it under the carriage with a couple of wing nuts, so I can swap extruders very easily.
• I brought the front of the bearing holder forwards 2mm. That stops the idler closing fully, which  makes it easier to feed in new filament and allows the hobbed bolt to be removed without having to remove the idler. The downside is it would be less tolerant of smaller diameter hobbed bolts.
• I made the idler bolt holes slightly further apart so that I could make them larger without intruding into the bearing holders.
• I added a slot around the top of the hole for the insulator. When it was simply a blind hole it had radiused corners at the end due to the fact that the filament has a minimum bend radius. That meant that, unless the insulator was chamfered, it did not go all the way to the end of the hole.

I use hobbed bolts and 10mm hot ends from  reprap-fab.org. Wolfgang makes the bolts so that the big gear can be spaced off from the bearing with 5 washers. That allows the small gear to be placed the right way round, allowing the big gear to be removed easily. M8 washers can vary in thickness so I made a printed spacer 7.5mm long to replace them.

I don't use Greg's accessible version of the extruder because I never remove the idler. Once I have got the spring tension correct I don't like to change it. If I need to clean the hobbed bolt I simply reverse out the filament, remove the nut and then remove the big gear and hobbed bolt. It only needs cleaning if there has been a malfunction due to a filament tangle or a nozzle blockage. 

To make the nut easy to remove, rather than use lock nuts or a Nyloc, I use a single nut and a weak spring. The spring stops the nut vibrating loose and gives enough pressure to keep the bolt in the correct position but it can be removed without using a spanner.

The extruder is the only part of the machine that wears out, so I have made it easy to swap out by adding a 9 way D type connector. D connectors screw together and have good strain relief for the cable, so they are reliable when subjected to constant movement. They are also rated for 5A per pin and 125°C, which is a good margin for this application.

I attach the connector with a bracket that is screwed to the motor by removing two of the motor's screws and replacing them with screws that are 5mm longer.

I have several extruders with difference nozzle sizes that I can change very quickly.

Mendel90

I never understood why Mendel has a triangular prism frame. The way I see it, the frame only has two functions: - To hold the Y bars in a flat plane and to support the tops of the Z bars. It isn't good at doing either:

• The main forces on the Z bars are in the direction of the X-axis and the frame has no strength in that direction. It wobbles when the X-carriage changes direction. 
• It also doesn't ensure the Y bars are in a flat plane because there is nothing to ensure one end triangle is not rotated slightly relative to the other. 

After a trip down a cobbled street in Sheffield my Mendel behaves as if one corner of the bed is lower than the other three. This is impossible because it has a flat sheet of glass on it, but it isn't obvious what needs to be adjusted to fix it but it must be the ends of the Y -bars. The bed needs to be level to within about 0.05mm for good results printing 0.3mm layers without a raft. That is difficult to achieve when the Y axis is strung from bars at opposite sides of the machine.

Other problems are: -

• It gets smaller at the top, so the maximum Z travel is limited by the extruder colliding with the bars. 
• The sizes of the Z axis and the Y axis are tied together, so you can't change one without the other. 
• It is difficult to adjust the axes so that they are orthogonal to each other and keep them that way if the machine is moved.

This machine is my attempt answer to these problems. I am calling it Mendel90 as I can't think of a better name at the moment. The 90 is to emphasise that the frame is based on right angles rather than 60 degree triangles.

Two flat sheets are mounted at right angles to form the XY and XZ planes. Two buttresses maintain them at right angles to each other. This relies on the sheets being cut at perfect right angles but in the UK you can buy sheet materials such as MDF or acrylic cut to size and they have good right angles. The only cutting I had to do was to cut the arch out with a jig saw. It doesn't need to be accurate and it could be done with a hand saw. The piece removed could be used to make the Y carriage, depending on the material.

The buttresses are bigger than they need to be. I took them all the way back to give me plenty of room  to mount my non-standard electronics, but it also has the advantage that the machine will sit on five of the six faces, making it easy to work on.

If the anti-backlash springs are fitted to the Z-axis it should print in all those orientations as well, which would be interesting to try. When printing directly on glass, parts come loose when the bed cools. If the machine was on its back they would fall out the bottom. Who needs an ABP? It might also solve the PLA ooze during warm up problem.

The gantry could be unscrewed and laid on its back over the top of the Y axis to make the machine more compact for travelling. In this case the buttresses could be slimmer to allow it to become even more compact.

I used B&Q style fixing blocks to fasten the sheets together.

I bought some of these and I printed some. They are a lot faster to print than Mendel frame vertexes! The economics are interesting: they are cheaper to print than buy, but while my machines are fully occupied making parts to sell, it is more economical for me to buy them. The printed ones are actually more accurate than the injection moulded ones! The holes are all over the place. I think they must be formed by removable cores and the tool must be worn allowing them to move.

I drilled pilot holes using a paper template. I did this by exporting DXF files of the sheets from OpenScad. I then hacked together a Python DXF reader and an SVG writer to make a program that generated drill centres. I printed them on a large plotter but it could be done with A4 sheets tiled together like the Darwin bed template.

The design is modelled in Openscad, down to the nut and bolt level, and is fully parametric so you can make any size machine and scale the rod diameters and motor sizes if necessary. The only limits are that eventually belts would need to be replaced by rack and pinion above a certain length. It also automatically generates a complete bill of materials for anything in the model.

See also: mendel90-extruder, mendel90-axes and mendel90-finishing-touches.


Merry Christmas!

Half belt hack

I found that I didn't have enough belt to complete the x-axis of my Prusa, but I did have a couple of offcuts about half the required length. Since less than half the belt actually passes over the motor pulley I simply joined them in the middle. My first idea was to print a two part clamp. Another idea was to use heat shrink sleeving, but in the end I simply tied them with some wire.

I joined them back to back so that the teeth mesh, keying them together. This has the beneficial side effect that the smooth part of the belt goes round the smooth idler pulley.

It might actually be worth doing this to get smoother running, even if you do have a belt long enough. Also if you are on a tight budget the second half does not need to be toothed belt at all. It could be packaging strapping or steel wire, etc.